As communication technology develops, services provided in mobile communication systems are evolving to include a multimedia broadcast communication service capable of supporting multimedia packet services to enable the transmission of large amounts of bulk data simultaneously to many User Equipment (UE). In order to support the multicast broadcast communication the 3rd Generation Partnership Project (3GPP) has been proposed a Multimedia Broadcast/Multicast Service (MBMS) in which one or more multimedia data resources provide a service to a plurality of UE.
Proposed MBMS services can transmit the same multimedia data to a plurality of UEs through a wireless Orthogonal Frequency Division Multiplexing (OFDM) network. MBMS services will be able to save wireless transmission resources by allowing a plurality of UEs to share one radio channel. MBMS services are intended to support the transmission of such multimedia data, as real-time image and voice, still images and text.
The basic time unit for the transmission and multiplexing signals—including data, control and reference signals—in OFDM system is an OFDM symbol, which consists of a cyclic prefix (CP) followed by an useful OFDM symbol. The useful OFDM symbol is the sum of multiple sub-carriers, each capable of carrying one modulation symbol which is referred to as a resource element (RE) in current 3GPP standards. An RE is basic frequency unit for signal transmission and multiplexing in OFDM system.
Current 3GPP standards specify normal and extended CP lengths for attaching to the useful OFDM symbol to avoid multipath interference at the UE. An OFDM symbol with a normal-length CP (OFSN) can be used for transmitting a signal requiring small or medium coverage to minimize CP overhead. An OFDM symbol with an extended-length CP (OFSE) can be used for transmitting a signal which requires large coverage to avoid multipath interference at a geographically remote UE.
When MBMS services are delivered to a single frequency network (SFN) MBMS service delivery area, the same MBMS signal is transmitted synchronously in time, using the same frequency sub-carriers, from within all cells in the MBMS service delivery area. Since an MBMS signal has a large coverage area, including multiple cells, OFSE is normally required for transmitting the MBMS signal.
3GPP normally use the term “unicast signal” to distinguish the cell specific signal (i.e. the signal that is generally different between the cells) and “MBMS signal” which can be the same from multiple cells. The CP length that is normally used for unicast signal transmission in a cell is called default CP of that cell.
3GPP standards define a transmission unit called slot which has length of 0.5 ms and consist of 7 OFSNs or 6 OFSEs. A sub-frame, consists of 2 slots, is currently assume to be smallest scheduling unit for transmission and multiplexing of unicast and MBMS signal at the physical layer. A slot or sub-frame can be viewed as a two dimensions grid (time and frequency) of multiple RE.
3GPP standards also assume that some types of Unicast signals, such as the L1/L2 control for Uplink (UL) scheduling, ACK for UL package transmission, Reference Signal for measurement, data for broadcast channel and paging channel, synchronisation signal, etc., need to be multiplexed with the MBMS signal in the same sub-frame. However, it is unclear how this multiplexing is done. 3GPP standards seem to assume that RE of some types of unicast (e.g. reference signal) and MBMS signals may be multiplexed in the same OFSE. If this is the case, it may happen that in a cell with normal CP being the default CP, same types of unicast signal can be transmitted in either OFSN or OFSE at any time. Therefore, it is unclear how all UEs, receiving unicast signal, are to be made aware of the dynamic change of CP length, between those OFDM symbols in which only Unicast signals are transmitted and those OFDM symbols in which MBMS and Unicast signals are multiplexed, in order to enable UEs to detect the useful part of a OFDM symbol transmitted after the normal or extended CP.
It would be desirable not to multiplex the MBMS signal and unicast signal in the same OFSE. If it is not possible, it would be desirable to provide a method of enabling a cyclic prefix length to be determined in an OFDM communication system in which the cyclic prefix length can vary dynamically. It would also be desirable to provide a method of enabling a cyclic prefix length to be determined in an OFDM communication system that ameliorates or overcomes one or more disadvantages or inconveniences of know cyclic prefix length determination methods.